Department of Animal and Plant Sciences, University of Sheffield, UK.
Edward Grey Institute, Department of Zoology, University of Oxford, UK.
J Hered. 2021 May 24;112(3):250-259. doi: 10.1093/jhered/esab016.
The sex chromosomes often follow unusual evolutionary trajectories. In particular, the sex-limited chromosomes frequently exhibit a small but unusual gene content in numerous species, where many genes have undergone massive gene amplification. The reasons for this remain elusive with a number of recent studies implicating meiotic drive, sperm competition, genetic drift, and gene conversion in the expansion of gene families. However, our understanding is primarily based on Y chromosome studies as few studies have systematically tested for copy number variation on W chromosomes. Here, we conduct a comprehensive investigation into the abundance, variability, and evolution of ampliconic genes on the avian W. First, we quantified gene copy number and variability across the duck W chromosome. We find a limited number of gene families as well as conservation in W-linked gene copy number across duck breeds, indicating that gene amplification may not be such a general feature of sex chromosome evolution as Y studies would initially suggest. Next, we investigated the evolution of HINTW, a prominent ampliconic gene family hypothesized to play a role in female reproduction and oogenesis. In particular, we investigated the factors driving the expansion of HINTW using contrasts between modern chicken and duck breeds selected for different female-specific selection regimes and their wild ancestors. Although we find the potential for selection related to fecundity in explaining small-scale gene amplification of HINTW in the chicken, purifying selection seems to be the dominant mode of evolution in the duck. Together, this challenges the assumption that HINTW is key for female fecundity across the avian phylogeny.
性染色体通常遵循不寻常的进化轨迹。特别是，在许多物种中，性限染色体经常表现出较小但不寻常的基因组成，其中许多基因经历了大规模的基因扩增。尽管最近的一些研究表明，减数分裂驱动、精子竞争、遗传漂变和基因转换在基因家族的扩张中起作用，但这些原因仍然难以捉摸。然而，我们的理解主要基于 Y 染色体研究，因为很少有研究系统地检测 W 染色体上的拷贝数变异。在这里，我们对鸟类 W 染色体上扩增基因的丰度、可变性和进化进行了全面研究。首先，我们量化了鸭 W 染色体上基因的拷贝数和可变性。我们发现了有限数量的基因家族，以及鸭品种之间 W 连锁基因拷贝数的保守性，这表明基因扩增可能不像 Y 染色体研究最初表明的那样，是性染色体进化的一个普遍特征。接下来，我们研究了 HINTW 的进化，HINTW 是一个突出的扩增基因家族，据推测在雌性生殖和卵母细胞发生中起作用。特别是，我们通过比较不同雌性特异性选择制度下选择的现代鸡和鸭品种与其野生祖先，研究了 HINTW 扩张的驱动因素。尽管我们发现选择与鸡的 HINTW 小尺度基因扩增相关的潜在因素与繁殖力有关，但纯化选择似乎是鸭的主要进化模式。总之，这挑战了 HINTW 在整个鸟类系统发育中对雌性繁殖力至关重要的假设。
